1. Field of the Invention
This invention relates generally to oscillator type electrical signal generators and more particularly to a microwave oscillator having a resonator portion operating at microwave frequencies and a signal amplification portion operating at an intermediate or IF frequency.
2. Background Art
There now exists a need in radar, communications and other types of electrical signal systems for highly stabilized oscillators which characteristically exhibit extremely low noise. Low noise performance typically requires some type of high Q circuit or some form of phase locking.
It is generally known that crystals, surface acoustic wave devices, bulk acoustic wave devices, inductance-capacitance and cavity resonators can be utilized to stabilize and control the oscillator. However, crystal and inductance-capacitance resonators normally have a low operation frequency and high instability due to mechanical vibration or jittering, respectively. Cavity resonators typically have a distinct limitation due to their relatively large physical size and the existence of multiple electromagnetic modes.
In an effort to increase the frequency of operation, UHF surface acoustic wave devices and bulk acoustic wave devices have recently been developed. It is known that bulk acoustic wave resonators can be made to operate in the GHz range; however, such structures support a train of discrete operating modes. One known technique for overcoming the undesirability of mode jumping has been achieved by high-Q multi-mode resonator controlled source having an oscillator sustaining stage circuit where the oscillator positive feedback loop contains a bulk acoustic wave resonator. The feedback loop circuit in such an oscillator includes a high and low frequency portion with the requisite frequency downconversion and upconversion being performed using separate frequency mixers whose local oscillator input drive are provided by an external signal generator. The high frequency portion of the loop, which includes the bulk acoustic resonator, operates at a desired output frequency which is equal to the sum of the local oscillator signal frequency and the frequency of the low frequency portion of the loop. A first mixer receives the output frequency and the local oscillator frequency and produces an IF frequency in the low frequency portion of the loop while a second mixer receives the frequency from the local oscillator and the IF frequency and produces an output of the output frequency in the high frequency portion of the loop. Thus as the local oscillator output frequency varies, the intermediate frequency in the low portion of the frequency loop varies inversely so that the up-down conversion performed by the system maintains the desired output frequency at a constant value. In order to prevent mode jumping, however, a narrow bandpass IF filter is connected in the low frequency portion of the loop.
In the prior art, signal amplification is provided in both the microwave and IF portions of the oscillator loop, and the narrow band IF filter is a requisite circuit element for prevention of mode jumping.
In the present invention, which also utilizes frequency conversion in the oscillator loop, the purpose of the embodiment of the circuit to be described is, specifically, to provide a low noise oscillator signal output by necessarily avoiding the use of both microwave signal amplification and narrow band IF filtering.